This invention relates to devices for blending materials eg mixing components of a mixture or homogenizing single components. The materials blended may be liquids or semi-liquid matter, and in some cases solids, powders or even gases. The invention is particularly concerned with the preparation of samples for bacteriological or chemical testing, but may also be used for the preparation of blended foods for example batters, pastries, sauces, baby foods and for the mixing of glues or paints or animal feeds additives.
A known blending device for preparation of bacteriological samples uses paddles and is described in GB 1 402 538. In use the specimen and diluent are contained in a sterile plastic bag and the paddles apply forces to the outside of the bag. This means that after processing the blender apparatus does not need to be cleaned and sterilized in preparation for further specimens, and so prevents cross contamination. This device includes two rectangular paddles of identical size and shape. The forces they impart to the sample are, for the most part, crushing. The reciprocating action of the paddles squeezes the specimen from side to side horizontally within the bag so mixing the resulting debris.
An examination of the fluid dynamics of this arrangement by the applicants has shown that the mixing action is most effective at the bottom of the bag and is reduced towards the top with an area between the tops of the paddles where the homogenization is minimal compared to the rest of the bag. Moreover we have appreciated that movement of liquid within the bag is important as it provides a form of washing action. The extraction of organisms requires the crushing or squeezing action and the washing action to not only release the organisms from the sample but also to drive them in to suspension. The resulting suspension must be as homogenous as possible and as representative as possible of the material under test.
It is an object of the present invention to provide an improved blender which alleviates the problems of the prior art.
According to the present invention there is provided a device for blending materials comprising a carrier support arranged to support a closed bag containing a material to be blended, a kneading means which is arranged to apply a kneading action to the bag walls for homogenising its contents, and holding means for holding the closed bag against the carrier support during kneading, the kneading means being arranged to act in conjunction with the carrier support to cause the contents of the bag to circulate during kneading.
Circulation of the contents is advantageous because it can provide more uniform or effective mixing and homogenization.
The holding means may comprise clamping means. When open ended sample bags are used the clamping means can be arranged to provide a temporary seal at the open end.
The kneading means may comprise at least one paddle arranged to act on the bag. Where a plurality of paddles are provided, the paddles can each be arranged to act on a different area of the bag, preferably while mutually out of phase.
Where the kneading means comprise at least one paddle said paddle can be shaped and dimensioned to cause and/or encourage circulation. Generally there should be a pair of paddles which are asymmetrically arranged with respect to each other in a manner which promotes circulation.
Each paddle may have a kneading surface which acts on the bag during kneading. The kneading surface can be shaped to cause and/or encourage circulation. In particular, the kneading surface may have a broad end and a narrow end. The displacement caused by the paddle at the broad end will then be greater than at the narrow end. This will tend to cause the contents of the bag to move in a direction from the broad end to the narrow end as the kneading surface acts on the bag.
Each paddle can be arranged so that some portions of the kneading surface contact the bag before other portions of the kneading surface during kneading. This may cause/encourage movement of the contents.
A pair of paddles can be provided in an opposed relation such that the broad end of each paddle is arranged to act on the bag at a region adjacent to a region at which the narrow end of the respective other paddle is arranged to act. Arranging such a pair of paddles to act on the bag in anti-phase is one way to produce a circulation of the contents of the bag.
The kneading surface may be generally triangular. Each side of the generally triangular shape may be curved.
An island baffle can be provided and arranged to cause, encourage or allow circulation around the baffle to occur. The island baffle can be maintained in a fixed relation to the bag. Where a pair of paddles are provided the island baffle can be located between the paddles. Facing surfaces of the island baffle and one or more associated paddle can be spaced from one another. The spacing at the broad end of the paddle can be less than at the narrow end. The facing surfaces of the baffle can be convex, the facing surface of the paddle can be concave.
Preferably the arrangement is such that extruding of the material occurs as it circulates.
Preferably one end of the island baffle is spaced from one internal boundary of the bag so that a first gap is defined between that end and the respective boundary, while the other end of the island baffle may be spaced from an opposing internal boundary of the bag so that a second gap is defined between said other end and said opposing boundary.
Preferably the apertures are arranged so that when material is circulating, during kneading, the material passes through one gap in a first direction and through the other gap in an opposing direction.
Preferably the material is extruded on passing through each gap.
A lower internal boundary such as a lower end seal of the bag in a plane of circulation may be arcuate, and preferably is semi-circular. This can encourage circulation. The curvature of the boundary may closely match a curvature of another facing surface or surfaces of one or more paddle. The boundary and said other facing surface(s) will usually be substantially parallel.